We have elucidated a series of biochemical steps in the activation of murine lymphocytes by mitogenic lectins. This biochemical pathway is apparently common to the activation of both T and B lymphocytes by a number of stimulants, including LPS, A23187, antiimmunoglobulin and soluble factors. This year's modest efforts conclude the first stages of the study, those assaying large numbers of lymphocytes. The next experiments are planned to analyze the early events in individual cells, through the use of fluorescent probes and flow microfluorometry. This refinement is necessary to establish the pathways followed by each cell type and the points at which intercellular interactions and communications (soluble factors) occur. We have also found that an antibody directed against lipomodulin, a cell surface inhibitor of phospholipase A2 (one of the enzymes in the common activation pathway) blocks the con A stimulation of murine thymus cells. In addition, this antibody also inhibits, probably by a cytotoxic mechanism, the development of suppressor T cells.